At irregular intervals, an influenza A virus emerges which is different from the current human seasonal influenza viruses and can not only infect humans but can also cause disease in some of them and crucially is capable of efficient human to human transmission. The virus has to be novel enough to prevail over the seasonal A viruses, and because of its novelty there can be little specific immunity among humans, except for older people who may have met a similar virus in the past. This new virus can then spread rapidly from human to human all over the world. Because of the lack of human immunity the virus causes a variable amount of severe disease and deaths: this is an influenza pandemic. Read more facts about pandemic influenza
Other types of influenza
Avian influenza is an infectious viral disease in birds, including domestic poultry. Avian influenza is mainly found in birds, but under certain circumstances infections can also occur in humans even though the risk is generally very low.
Seasonal influenza is a preventable infectious disease with mostly respiratory symptoms. It is caused by influenza virus and is easily transmitted, predominantly via the droplet and contact routes and by indirect spread from respiratory secretions on hands etc.
Swine influenza is a respiratory disease of pigs caused by type A influenza viruses that regularly cause outbreaks of influenza in pigs. Influenza viruses that commonly circulate in swine are called “swine influenza viruses” or “swine flu viruses.” Like human influenza viruses, there are different subtypes and strains of swine influenza viruses.